Patent GB-667196-A (GB)
Title: Speech communication system
IPC: H04B10/00
Filed: 1949-05-20
Abstract: 667,196. Secret transmission. STANDARD TELEPHONES & CABLES, Ltd. May 20, 1949 [July 24, 1948], No. 13519/50. Class 40 (iv). [Also in Group XL (b)] To transmit a complex wave, its fundamental frequency (which may vary) is developed from the wave together with a control voltage which varies in accordance with the frequency of the fundamental, this control voltage being applied to maintain a plurality of filters tuned to the fundamental frequency and its harmonics, to which the complex wave is applied, and the outputs of the filters, i.e. voltages representing the amplitudes of the fundamental and its harmonics, together with the control voltage, are transmitted for reconstitution at the receiver ; these signals are stated to vary comparatively slowly, e.g. at 25 c/s., so that if thirty harmonics are used, a bandwidth of 30Î25=750 c/s. is stated to be sufficient for the transmission of intelligible speech. The transmitter shown in Fig. 1 shows a speech source 1 feeding a pitch meter 3 and harmonic filters 2; the pitch meter produces two signals, viz. a sinusoidal oscillation at the fundamental frequency, which is transmitted from 6, and a voltage, whose amplitude is dependent on the fundamental frequency, which is applied to control the pass-band of the filters 2 to maintain them tuned to the harmonics. The outputs of the filters 2 are rectified and transmitted. If the speech source is an unvoiced wave, the pitch meter is arranged to produce a frequency of 50 c/s. The signals are transmitted by any suitable means, and at the receiver Fig. 2, the fundamental is isolated and applied to the tuning control 11 which feeds a harmonic generator 14 feeding harmonic filters 17. The tuning control also produces a voltage, whose amplitude is dependent on the fundamental frequency, which is applied to the filters to maintain them in tune with the harmonics. If the signal corresponds to unvoiced speech, the tuning control 11 feeds a noise source 16. The outputs of the filters 17 are modulated in amplitude at 10 by the appropriate received signal and the speech signal is reconstituted by combining their outputs. Filter.-The signal is recorded on a strip 19, Fig. 3, moving between a light source 21 and a light cell 25, a graded screen whose opacity is appropriately varied according to the harmonic being interposed. The output is stated to be equivalent to the output of a tuned circuit, the tuning being varied by varying the speed at which the record moves. Recorder.-An aluminium strip 26, Fig. 5, is set into vibration at 29 by the speech signals, so that waves travel down the strip. The tension is varied by applying the voltage corresponding to the fundamental frequency to a solenoid 23 acting on a plunger 28. Light from a source 34, Fig. 6, is reflected from the strip 26 to fall on a screen 36 of uniformly graded opacity so that on the screen is produced a curve whose intensity varies with the intensity of the input. Complete installation.-A plurality of speech signals 37, Fig. 7, are applied to recorders 39 of the kind described above, the recorders being gated successively from a pulse source 40. Light from each recorder is reflected from mirror 41 on to a bank of filters 44 which cooperate with light sensitive cells 45 to form harmonic filters as described above. The output of each cell is transmitted. At the receiver, Fig. 9, the received signals are applied to light sources 51, one corresponding to each cell of Fig. 7. The light is then reflected from rotating drum 56 to phosphor strips 57, one to each voice channel. The light from strips 57 acts as source 34 in Fig. 6 and in conjunction with the appropriately energized aluminium strips, modulates the harmonics to give appropriate outputs in photo-cells receiving light from the energized strips. These outputs are combined to give speech signals. Pulse modulation.-The amplitudes of the harmonics may be transmitted as three unit pulse codes while the frequency of the fundamental may be transmitted as a seven unit pulse code. The Specification as open to inspection under Sect. 91 includes the following subject-matter: Visual speech signals.-Voice signals are applied to the recorder 60, Fig. 10 (Cancelled), co-operating with the harmonic filters comprising opaque screens 62 and cells 63; each cell controls a light source 64 illuminating a fluorescent travelling band 65. This subject-matter does not appear in the Specification as accepted.